1. Field of the Invention
The present invention relates to sonar signal processing and more particularly to an active sonar waveform transmission and receiver processing system which improves performance by: reducing the effects of reverberation, improving active classification without requiring a unique transmission which could "alert" the target, reducing the effects of Mutual Interference (MI) and improving immunity to ping stealing by enemy submarines, thereby achieving greater performance in all these areas concurrently. Mutual Interference is caused by a transmission of one ship sonar system being received unintentionally by another ships system. Consorts in the same area find this to be a common problem which causes both marking and blanking of displays. Ping stealing is a function carried out by a target to derive information about the course, speed, mode, depth, etc., of the ship or submarine whose transmissions are being received and analyzed.
2. Description of the Prior Art
Current active sonar transmission/reception techniques all have drawbacks in one performance area or another. Improvement in a selected area requires compromised or reduced performance in other areas. Present techniques however do not permit improvement in all areas simultaneously. For example, although widening the transmitted FM bandwidth of a pulse reduces reverberation, it also causes separating of the target into individual lower strength reflectors thus causing increased "splitting losses", and reduced immunity to MI and ping stealing. Currently Frequently Shift Keying (FSK) employs subpulses of longer than 100 ms with subpulse frequency separation set in a geometric pattern. Using Frequency Shift Keying (FSK) with typical Continuous Waveform (CW) subpulses degrades range resolution/ classification and produces higher reverberation than that due to FM. Also, all FM, CW and FSK transmissions currently used for long range search or classification experience signal-to noise reduction due to overlapping returns from the major reflectors of a target, most of which are spaced within 100 ms.